820 EMBRYOLOGY 



lages are not divided at the lines of separation of the somites, but the 

 somites fuse together and the cartilages which are to be developed into 

 the bodies of the vertebrae are so divided off that one cartilage occupies 

 the place of the adjacent halves of two somites. Between the bodies 

 of the vertebrae, the notochord presents regular enlargements sur- 

 rounded by a delicate membrane. As ossification of the spinal column 

 advances, that portion of the notochord which is surrounded by the 

 bodies of the vertebrae disappears, leaving the enlargements between 

 the vertebrae distinct. These enlargements, which are not permanent, 

 are gradually invaded by fibrous tissue, their gelatinous contents disap- 

 pear, and the intervertebral disks, composed of fibro-cartilaginous struc- 

 ture, remain. These disks are permanent between the cervical, the 



dorsal and the lumbar vertebrae ; but they 

 eventually disappear from between the differ- 

 ent parts of the sacrum and coccyx, as these are 

 consolidated, this occurring, in the human sub- 

 ject, between the ninth and the twelfth years. 



Vertebral Column, etc. In Figs. 229 and 

 230 (C, C), are seen the two masses of cells 

 (somites) situated by the sides of the neural 

 canal. Cells of the inner borders of these 



Fig. *&.- Human embryo, b dieS eXtCIld ai " Und and encroach On the 



about one month old, showing the notochord and form the bodies of the verte- 



large size of the head and upper , -p,, , , .. , , 



parts of the body, the twisted form br3e - They also extend over the neural canal, 

 of the spinal column, the rudi- closing above, and their processes are called 



mentary condition of the upper 



and lower extremities and the the medullary, or dorsal plates. From the 



rudimentary tail at the end of sides f tfa bodies f tfa ver tebr3e, the Various 

 the spinal column (Dalton). 



processes of these bones are formed. As the 



spinal column is developed, its lower portion presents a projection 

 beyond the pelvis, which constitutes a temporary caudal appendage, 

 curved toward the abdomen ; but this no longer projects after the bones 

 of the pelvis are fully developed. At the same time the entire vertebral 

 column is curved toward the abdomen, and it is twisted on its axis from 

 left to right, so that the anterior face of the pelvis presents a right angle 

 to the upper part of the body ; but as the inferior extremities and the 

 pelvis are developed, the spine becomes straight. The vertebrae make 

 their appearance first in the middle of the dorsal region, from which 

 point they rapidly extend upward and downward until the spinal column 

 is complete. 



At the base of the skull, on either side of the superior prolongation 

 of the notochord, are two cartilaginous processes that are developed 

 into the so-called cranial vertebrae. In this cartilaginous mass, three 



